At sufficient intensities, low-wavelength EM radiation registers an effect on the detector chip. This has been demonstrated with Sunlight, a Carbon Filament bulb, a 632nm red laser, a red LED and a blue LED. Increasing the IKrum value of the chip decreases this effect markedly; however, this also significantly reduces sensitivity. Over the last two years, we at Simon Langton ran a number of experiments into these effects, the most significant of thes

Red Laser

A class 2 632nm was targeted at a Mk-1 Timepix detector chip. The chip was switched on, followed by the laser, which was allowed to run for 5 seconds before it was switched off before the chip had finished its capture. This is because a much larger reading is registered during the switch-on and switch-off than inbetween; it is thought that this is because of the IKrum mechanism in the chip responding more to sudden changes in light than constant sources, but the possibility of it being caused by a surge of intensity of the laser light cannot be ruled out yet. Frames are one second long.

Readings were taken for IKrum=1 and IKrum=5. When IKrum was set to 1, an effect was registered inbetween switch-on and switch-off, so all frames taken are presented. When IKrum was set to 5, an effect was only registered when the laser was switched off, so only this frame is presented.

IKrum=1

Switch-on.

Switch-off.

IKrum=5

Switch-on.

Conclusions

This experiment shows that visible light can have an effect on the chip, owing to the very small range of wavelengths in laser light. This effectively eliminates the possibility that sunlight's effect on the chip is purely down to its UV component.

Light-Emitting Diodes

This experiment was run confirm that pure visible non-coherent light, and to try and test which wavelength of visible light has the greatest effect. The Mk-1 Timepix chip was exposed to red and blue LEDs, focused into a circle on the chip by a pair of lenses. The intensity of the incident light was measured and slowly increased, and the lowest intensity at which an effect could be registered on the chip was measured.

Precise results of the experiments are currently lost somewhere in the labyrinthine computer systems of the Star Centre. The results essentially indicate that red light has a slightly greater effect on the chip than the same intensity of blue, but there were too many variables which are too difficult to precisely control to make the conclusions too reliable (for instance, the diameter of the circle of light focused on the chip). We suspect that heating effects are to blame, but cannot say for certain. Hopefully in the future the precise results will emerge. Further investigation might well be useful.